Field of the Invention
The present invention relates to a method, a device and the like for driving a capacitance transducer used as an ultrasound conversion element.
Description of the Related Art
Conventionally, micro mechanical members manufactured by micro machining techniques can be worked in the order of micro meters, and using such micro mechanical members, various micro functional elements are provided. Capacitance transducers formed using such techniques have been studied as alternatives to piezoelectric elements. These capacitance transducers enable transmission and reception of acoustic waves such as ultrasound waves (hereinafter, sometimes represented by ultrasound waves) using vibration of a vibration membrane facilitates provision of excellent wideband characteristics particularly in liquids. In the present specification, acoustic waves include what are called sound waves and ultrasound waves.
Ultrasound diagnostic apparatuses are apparatuses that transmit ultrasound waves from a capacitance transducer to a test object, receive reflected signals from the test object via the capacitance transducer and pick up ultrasound images based on the received signals. International Publication No. WO2009/075280 proposes suppression of reduction in sensitivity of the capacitance transducer due to a collapse state. Furthermore, Japanese Patent Application Laid-Open No. 2006-122344 makes a proposal relating to a method for driving a capacitance transducer, for an increase in the sound pressure of transmitted ultrasound waves and enhancement in the efficiency of reception of reflected signals.
A capacitance transducer is configured by a plurality of elements each including cells each having a structure in which a vibration membrane including one electrode of a pair of electrodes provided with a cavity therebetween is supported in such a manner that the vibration membrane can vibrate. The elements may vary in characteristics because of, e.g., variation in film thickness that occurs at a time of manufacture. If acoustic waves are transmitted by applying a same bias voltage to the plurality of elements and also applying a same transmission drive voltage to the plurality of elements, variation may occur in the strength of acoustic waves transmitted from the elements in the single capacitance transducer. The variation in the strength of transmitted acoustic waves causes variation in reflected waves from a test object, which may result in the distortion of the ultrasound images based on the received signals and/or a decrease in resolution.
As with the technique described in International Publication No. WO2009/075280, if transmission and reception are performed with a high bias voltage applied, the variation in the strength of transmitted acoustic waves may be larger because of nonlinear acoustic wave strength characteristics of the capacitance transducer. Furthermore, in the technique of Japanese Patent Application Laid-Open No. 2006-122344, the sensitivity of reception of reflected acoustic waves is adjusted by changing a bias voltage in a step-by-step manner in transmission and reception, which, however, cannot be said that the capacitance transducer is driven with the variation in the characteristics of a plurality of elements taken into account.